Ultra-wideband (UWB) communications systems are normally defined as carrier-less communications systems wherein the bandwidth of the signal being transmitted, fB, is greater than or equal to 0.20 fc, where fc is the center frequency of the signal being transmitted. Additionally, the UWB communications system should have a minimum bandwidth of 500 MHz. Note that the definition for UWB communications systems and devices is as defined by the Federal Communications Commission (FCC) of the United States. UWB communications systems have been around for a great number of years, and the majority of them fall under one type of system, they modulate a stream of short-duration pulses (with an approximate duration which ranges from 0.2 nanoseconds (ns) to 2 ns), either in time (pulse position modulation (PPM)), amplitude (pulse amplitude modulation (PAM)), or phase angle (bi-phase modulation).
The FCC, in Report Order 02-48 released in February of 2002, has specified a set of spectral allocation, technical standards, and operating restrictions for several different types of UWB devices. For example, in the Report Order, the FCC specifies that indoor UWB devices may operate within a frequency range of 1.9 to 10.6 GHz while hand-held UWB devices may operate within a frequency range of 3.1 to 10.6 GHz. Within the permitted frequency ranges, the FCC also places a limit upon maximum transmit power. According to the FCC, many types of communications systems can be classified as a UWB communications system if it conforms to the specified spectral allocation, technical standards, and operating restrictions.
A potential source of performance concern for a UWB communications system that makes use of the entire frequency range provided by the FCC (either 1.9 to 10.6 GHz or 3.1 to 10.6 GHz) is that since the frequency range is so large that there can be portions of the frequency range that is in use by other electronic devices. For example, in a frequency range commonly referred to as the Unlicensed National Information Infrastructure (UNII) band, which ranges from 5.15 to 5.825 GHz, there may be a wide variety of electronic devices. Devices operating in the UNII band can transmit at significantly greater power than UWB devices. Therefore, these devices can be sources of interference for UWB devices. Furthermore, UWB devices can also interfere with the UNII devices.
A proposed solution to interference with UNII devices (and other devices) is to use specially designed pulses with no transmission signal in the occupied frequency bands. The use of the specially designed pulses can allow existing UWB communications systems to operate with little to no interference with (and from) devices operating in the occupied frequency bands.
Another proposed solution to interference from other devices is to use a multiband variant of the UWB communications system. In this solution, the UWB frequency range can be divided into multiple frequency bands and the UWB communications system can transmit on different frequency bands as interference is detected.
One disadvantage of the prior art is the multiband solution may not be compatible with existing UWB communications systems. Therefore, in order to provide interference immunity, existing UWB communications systems may need to be replaced with new UWB communications systems that can transmit over different frequency bands.
A second disadvantage of the prior art is that the use of the specially designed pulses can make it difficult to permit multiple different UWB communications systems operating within the general vicinity to establish simultaneous but distinct communications networks since the transmission of the specially designed pulses by a single UWB communications system occupies the entire UWB spectrum and may prevent other UWB communications systems from transmitting.
Another disadvantage of the prior art is that the large operating frequency range (large bandwidth) of existing UWB communications systems can make it difficult to develop and test radio frequency hardware.